Process vessels construction materials

General Considerations Most pressure vessels for the chemical-process industry will continue to be designed and built to the rules of Sec. T11, Division 1. While the rules of Sec. T11, Division 2, will frequently provide thinner elements, the cost of the engineering analysis, stress analysis and higher-quality construction, material control, and inspection required by these rules frequently exceeds the savings from the use of thinner walls. 

Many applications of novolacs are found in the electronics industry. Examples include microchip module packaging, circuit board adhesives, and photoresists for microchip etching. These applications are very sensitive to trace metal contamination. Therefore the applicable novolacs have stringent metal-content specifications, often in the low ppb range. Low level restrictions may also be applied to free phenol, acid, moisture, and other monomers. There is often a strong interaction between the monomers and catalysts chosen and attainment of low metals levels. These requirements, in combination with the high temperature requirements mentioned above, often dictate special materials be used for reactor vessel construction. Whereas many resoles can be processed in mild steel reactors, novolacs require special alloys (e.g. Inconel ), titanium, or glass for contact surfaces. These materials are very expensive and most have associated maintenance problems as well. 

Process flow diagrams are more complex and show all main flow streams including valves to enhance the understanding of the process as well as pressures and temperatures on all feed and product lines within all major vessels and in and out of headers and heat exchangers, and points of pressure and temperature control. Also, information on construction materials, pump capacities and pressure heads, compressor horsepower, and vessel design pressures and temperatures are shown when necessary for clarity. In addition, process flow diagrams usually show major components of control loops along with key utilities. 

Atomic power production The necessity for avoiding contamination of operative liquids, together with other requirements which must be met in selecting constructional materials in this highly specialised field has resulted in the choice of austenitic steels for applications in heat exchangers, pressure vessels, pipelines and fuel processing. 

The need for temperature cycling should be taken into account when designing or conducting tests. The nature of the test vessel should be considered for tests in aqueous solutions at temperatures above about 60°C since soluble constituents of the test vessel material can inhibit or accelerate the corrosion process. An inhibiting effect of soluble species from glass, notably silica, on the behaviour of steel in hot water has been shown . Pure quartz or polymeric materials are often more appropriate for test vessel construction. 

These were plotted and are compiled in API RP 520, Chart D-2 (page 55). The data plotted is for vessels constructed of ASTM A-5I5, Grade 70 steel, a steel typically employed for process vessels. If other materials are used an allowance for their stress characteristics under heat application needs to be made. Therefore a general determination of the need for protective measures, such as depressurization, can be made for a particular vessel by comparison to the D-2 chart and selected fire exposure temperatures. It should be noted that this is the best available fire test exposure data in the public domain. Improved methods and test data may be available in the future to refine the calculation methods. 

Sound practice dictates that storage tanks and process vessels containing hazardous chemicals be properly designed and used only for their intended purpose. They should also have, as a minimum, overflow alarms. Once these conditions are met, it remains to ensure the structural soundness of the containers, to enforce proper hazardous materials handling procedures, and to construct and maintain secondary containment and collection facilities (Shields 1980). 

It has been estimated that the consumption of rubber in ebonite products increased on an average of about 6% a year until 1951 when different plastics appeared in the market as competitive materials [6]. But because of the superiority of ebonites in acid and chlorine duties, it still maintains its place in the process industries as a material of construction as evidenced by its increased use in anticorrosive lining of chemical process vessels and tanks for the process industries and rolls for steel and textile and paper and pulp industries. 

As with double-wall containment systems, a transfer vessel s construction materials, design pressure, and temperature rating should at least equal those of the equipment being protected. Construction materials can differ if the transfer vessel will only be exposed to the corrosive process for an acceptably short duration. 

Construction materials should be selected that will withstand long-term exposure to the chemicals used in the process. This includes gaskets, which tend to be weak points in piping and vessels. Gaskets are discussed in Section V,E of this chapter. Occasionally fire-safe emergency valves are installed with gaskets that will quickly burn away if there is a fire. This tends to defeat the purpose of fire-safe valves. 

Materials of Construction. For demanding SCF processes such as precision cleaning, the pressure vessel and most of the components are fabricated fi om 316 stainless steel. Process vessels... 

The material of construction for reactors, process vessels, heat exchanger tubes, and process pipelines can be similarly chosen from carbon steel, suitable grades of stainless steels, Teflon-lined steel, fibreglass, polypropylene, glass, etc., after laboratory tests at operating conditions to confirm suitability (for minimising chances of contamination). 

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